Download Neuroscience insights on variations by age v2

Neuroscience insights on variations by age
Supplemental report by John P. Eberhard, April 2007
A child’s brain also goes through “critical” periods of construction. The largest part of
the construction process of the new brain occurs in the early life of the fetus. During this
early stage, the basic structure of the brain is created and the sensory organs form their
basic connections along the nerve pathways. This is all done from the genetic encoding of
nerve cells and biological processes that guide the construction provided by DNA we
inherit from our parents (this encoding is like blueprints for a building).
The human body and brain are composed of cells. Each cell contains membranes, a
cytoskeleton, organelles, mitochondria, and a nucleus that contains our genes. These cells are
formed over the 40 weeks of gestation by the embryo. As the human body takes shape in the
womb, populations of cells enter different streams of development: one is destined to give rise to
the lungs, another to muscle, a third to the kidneys and bladder, etc. DNA provides instructions
for forming proteins from amino acids. Some proteins are structural while others are enzymes
made within the factory.
During the third week of life as an embryo, the first structure for the brain begins to form.
It continues to develop over the next eight months as cells are generated by the division of
progenitors (popularly called stem cells) that migrate into position and begin forming networks
by connecting with other neurons. There is no master plan or central processor involved; it just
happens as the nervous system organizes its own intricate structure.
The sensory systems of the human fetus develop in a predetermined sequence. Four of
them, including touch, pain, position, and temperature sensitivity are the first to appear. Shortly
after this, the “vestibular modalities” – the sensory systems of the middle ear that detect motion
and determine balance – are installed. Next come the chemosensory systems of smell and taste.
All of these systems are well established during the third to sixth month of fetal life. During the
sixth to ninth month in the womb the construction of the brain enters a critical stage, first for the
auditory cortex (hearing) and then for the visual cortex (sight). The development process
continues once a child is born, adding and subtracting neural networks based on experiencing the
world.
If a child is born prematurely, at six or seven months, the critical development
processes for hearing and sight can be disrupted by being exposed too early to noise and
light in the neonatal nursery. These systems will be forced to begin working prematurely.
As Dr. Stanley Graven (1992) has reported in his study of neonatal units, this does not
result in a child being born either deaf or blind, but they lose their acuity. This is a good
example of how knowledge from neuroscience can provide evidence-based design
criteria for building spaces.
The early brain and development
The brain we use as adults to remember the past is a different brain than the one that
stored memories when we were children. The chart shown below (from the New
Cognitive Neurosciences, page 85) shows the mean synaptic density in three areas of the
brain at various ages:
 In the auditory cortex – filled circles
 The calcarine (sulcus on both sides of which lies the visual cortex) – open circles
 The Prefrontal cortex – Xs
This chart is the result of having powerful brain imaging methods to study aspects of
the physiology of sensory and language processing in humans. Event-related brain
potentials (ERPs) are voltage fluctuations in the EEF (Electroencephalogram - a method
of brain scanning) in response to a controlled stimulus. The ERPS essentially show the
number of synapses in the neuronal populations that are recruited during the processing
of the stimulus. The chart shows the dramatic changes in synaptic populations during the
first two years of life when “pruning” of unused synapses occurs. As Prof Joan Stiles
says in her summary of these changes:
“Brain development is a complex and protracted process. It begins at 28 days
post-conception (in the embryo) and continues into adulthood. Both biology and
experience play critical roles in shaping the final organization of the brain. Development
is more that a simple unfolding of a predetermined genetic plan. While genes are
critically important for brain development, the development process is also adaptive. It
is the interaction of biological systems with each other and with input from the world
(such as experiences with life) that ultimately determines brain organization and
function.” Quoted in Rita Carter’s “Mapping the Mind”. Page 22
We also remember how to walk and talk – both of which we learned before we were
three – because these are procedural memories, not episodic memories.